When the screw stops rotating and moves backward, the mold clamping part begins to work to close the mold and the injection unit subsequently moves forward to make the nozzle and mold spruing gate joint with each other;the injection oil tank plunger drives the screw to move forward rapidly, and the injection pressure and speed as required by the molding technics inject the fused plastics into mold cavity. To avoid refluence of the materials injected into the mold and to supplement the small amount of materials needed due to the contraction caused by the cooling and molding of parts, the screw, upon completion of injection, shall still keep certain pressure on the fused materials, i.e. pressure maintaining, until the materials at the gate have cooled and molded.